The present invention relates generally to ceramic matrix composites, and more particularly to ceramic matrix composites reinforced with combinations of inorganic fibers and inorganic particles which exhibit enhanced interlaminar shear strength and other desirable properties.
The use of inorganic whiskers and fibers to reinforce glasses, glass-ceramics, and ceramics has long been practiced. The mechanism of strengthening of glass or ceramic bodies by fibers is considered to be that of load transfer by the matrix to the fibers through shear. This load transfer shifts stress from the glass or ceramic matrix to the relatively long, high modulus fibers, while the fibers at the same time may act to impede crack propagation in the matrix material. Whiskers are thought to impart strengthening by a similar mechanism, but load transfer to whiskers by the matrix is more limited due to the limited length and aspect ratio of the whiskers. Theoretically, a whisker which is sufficiently short will not be loaded to the breaking point by the matrix under stress, and therefore full advantage cannot be taken of the high strength of the whiskers.
Among the fibers and whiskers which have been suggested for use as reinforcement for non-metal matrix materials are silicon carbide, silicon nitride, alumina and carbon whiskers. For example, U.S. Pat. No. 4,324,843 describes SiC fiber reinforced glass-ceramic composite bodies wherein the glass-ceramic matrix is of aluminosilicate composition. U.S. Pat. No. 4,464,475 describes similarly reinforced glass-ceramics comprising barium osumilite as the predominant crystal phase, while U.S. Pat. No. 4,464,192 describes whisker-reinforced glass-ceramic composites of aluminosilicate composition.
A principal objective of whisker reinforcement in glass, ceramic and glass-ceramic materials for high temperature applications is that of increasing the toughness of the material. A toughened ceramic material exhibits improved resistance to cracking failure from flaws sustained in use, offering the possibility of increased fatigue lifetime As noted in U.S. Pat. No. 4,626,515, the addition of fiber reinforcement to glasses such as alkali-free alkaline earth aluminosilicate glasses can result in substantial strengthening, while whisker additions to those glasses were found effective to enhance the toughness of the glass.
Many of the fiber-reinforced composites described in the prior art are of laminar type, i.e., the fiber reinforcement is preferentially disposed in layers within the material, with the layers consisting of fiber groups or arrays wherein the fibers within each layer are principally disposed in substantially parallel alignment in a single direction, termed the fiber direction of the layer. Each such layer may be characterized as unidirectional in that the fibers in the layer will all be oriented in substantially the same axial direction (typically .+-.5.degree. ).
Ceramic matrix composites to be utilized in high-stress, high-temperature environments, will desirably exhibit not only high bending strength and fracture toughness, but also strength properties which are relatively isotropic, i.e., not confined to a single "strong" axis of the composite material. The attainment of such properties in laminar systems normally requires at least some cross-ply lamination of fiber reinforced laminae in the material since, as has been observed, whiskers alone cannot impart the necessary high isotropic flexural strength to the material.
In fiber-reinforced ceramic matrix composite of uniaxial fiber orientation, transverse flexural strengths, i.e., strengths in bending about axes parallel to the fiber direction, are generally at least two orders of magnitude lower than strengths in bending across the fiber direction. In cross-ply laminates, additional strength factors such as interlaminar shear strength must be considered. Stresses applied to the laminated structure in directions parallel to the planes of lamination give rise to shear stresses within interlaminar regions of the composite, which regions are not effectively fiber reinforced. These regions therefore exhibit relatively low strength and provide preferred paths for crack propagation, so that layer separation and delamination of the composite under stress can occur.
Attempts to improve such properties have resulted in the development of so-called hybrid ceramic composites, which are composites containing both fiber reinforcement and an added whisker phase. In U.S. Pat. No. 4,615,987, for example, a combination of fibers and whiskers was introduced into an anorthite (CaO--Al.sub.2 O.sub.3 --SiO.sub.2) glass-ceramic matrix to enhance physical properties, while in copending, commonly assigned patent application Ser. No. 47,128, filed May 8, 1987, whisker-containing fiber-reinforced ceramic matrix composites having a lithium aluminosilicate matrix reinforced with SiC or similar fibers are disclosed.
There are, however, some disadvantages associated with the inclusion of inorganic whiskers in fiber-reinforced ceramic matrix composites. For example, in some systems, the increases in transverse flexural strength and interlaminar shear strength which have been attained are partially offset by losses in flexural strength in the strong axis of the material. The mechanism for such strength losses have not been fully explained. Another disadvantage connected with the use of whiskers is the somewhat high cost of the commercially available whisker materials.
The process of crack propagation in systems comprising particulate rather than fibrous inclusions has also been the subject of study, with mixed results. One study of the fracture properties of glass-ceramics, reported by P. Hing et al. in "The Strength and Fracture Properties of Glass-ceramics", Journal of Materials Science, 8 (1973), pages 1041-1048, suggested an increase in strength and effective surface energy for crack initiation as the mean free path in the intercrystalline glass of a lithium disilicate glass-ceramic decreased. Further, G. C. Wei et al., in "Improvements In Mechanical Properties in SiC By The Addition of TiC Particles", Journal of the American Ceramic Society, 67 (8), pp. 571-574 (August 1984), found an increase in toughness and flexural strength with TiC additions to a silicon carbide system in proportions up to about 25% by volume.
On the other hand, T. Mah et al. in "Fracture Toughness and Strength of Si.sub.3 N.sub.4 Composites", Ceramic Bulletin, 60 (11), pp. 1229-1231, 1240, (1981), found a decrease in strength to accompany an increase in fracture toughness in a silicon nitride ceramic system containing a titanium carbide second phase. This was attributed to residual stresses in the two-phase composite, possibly accentuated by flaws developed at the interface between the silicon carbide matrix and titanium carbide agglomerates present therein. Similar results were reported by J. G. Baldoni et al. in "Mechanical Properties and Wear Resistance of Silicon Nitride-Titanium Carbide Composites", published in Tailoring Multiphase and Composite Ceramics, MATERIALS SCIENCE RESEARCH, Volume 20, R. E. Tressler et. al., Editors, Plenum, New York, 1987.
It is evident from the literature, therefore, that the strengthening or toughening of ceramic systems by the introduction of second phase particles therein, if any, is manifestly different for different systems, due to factors such as differences in matrix fracture energies, differing bond strengths between the additive and the matrix, and other variables. The prediction of changes in fracture energy by the addition of additives to any particular system is thus quite difficult, and this difficulty is further compounded if additional phases such as reinforcing fibers are present in the system. Nevertheless, because of the very wide range of possible applications, there is considerable interest in refractory ceramic materials exhibiting improved strength and toughness.
It is a principal object of the present invention to provide a novel fiber reinforced composite system wherein significant enhancements in fracture energy can conveniently and economically be provided.
It is a further object of the invention to provide a laminar fiber-reinforced composite system wherein improved transverse flexural strength and interlaminar shear strength can be realized.
Other objects and advantages of the invention will become apparent from the following description thereof.